1. Field of the Invention
This invention relates to an antenna unit installed in a small communication device of a mobile telephone, etc., and in particular to an antenna unit adapted to a communication device of a mobile telephone, etc., having an NFC (Near Field Communication) function and a mobile terminal using the antenna unit.
2. Description of the Related Art
Hitherto, for example, an IC card system has been widely known as a system for conducting proximity communications. In the IC card system, a reader/writer generates an electromagnetic wave, thereby forming an RF field (magnetic field). When the IC card is brought close to the reader/writer, the IC card receives power supply according to electromagnetic induction and transfers data to and from the reader/writer.
For example, NFC exists as a communication protocol for conducting proximity communications represented by the IC card system. In recent years, a mobile telephone has been caused to execute proximity communications in accordance with the NFC communication protocol and a communication device of a mobile telephone, etc., has been provided with a card function. Further, an art of reading information in a tag attached to a target through a mobile telephone using the NFC communication protocol is also proposed (reader function). Patent Document 1 discloses an example of an antenna unit for executing the NFC communication protocol.    Patent Document 1: JP2008-48376
However, to install an antenna unit in a mobile telephone, the antenna unit must also be small matched with the size of the mobile telephone. A chip antenna is considered as an antenna unit fitted for installation in a mobile telephone. To use a chip antenna, however, it becomes difficult to ensure a predetermined communication distance although there is a size merit. According to an experiment, to conduct proximity communications using a chip antenna, stable communications were able to be conducted only to a distance of about 5 mm. If proximity communications are conducted using a mobile telephone at this distance, in fact, communications cannot be conducted because of the thickness of the cabinet of the mobile telephone or the like. Even if an extremely thin cabinet is used, a mobile telephone and targets (reader/writer and tag) must be brought almost into contact with each other and the usability of the system is poor for the user.
As an antenna unit for executing NFC, it is desirable that proximity communications should be able to be conducted with spacing at a considerable distance (generally, about 30 mm). It is therefore an object of the invention to provide an antenna unit that can conduct proximity communications at a predetermined communication distance (30 mm) or more and has a size fitted to installation in a mobile telephone and a mobile terminal using the antenna unit.